What criteria should I use to decide when to refer a patient with PUO

Guideline-aligned answer with reasoning, red flags and references. Clinically reviewed by Dr Kola Tytler MBBS CertHE MBA MRCGP.

Criteria for referral of a patient with pyrexia of unknown origin to secondary care include:

Presence of any 'red' features indicating serious illness or immediately life-threatening conditions, which require urgent referral to a paediatric specialist or emergency care within 2 hours .
Presence of 'amber' features without a clear diagnosis, where clinical judgement should guide the urgency of referral or arranging further assessment by a paediatric specialist .
Infants younger than 3 months with fever should be referred for observation and investigations including full blood count, blood culture, C-reactive protein, urine testing, and possibly lumbar puncture, especially if they appear unwell or have abnormal white blood cell counts .
Children with fever who are shocked, unrousable, or showing signs of meningococcal disease require urgent review by a paediatrician and consideration for intensive care referral .
If no diagnosis is reached but the child remains ill longer than expected for a self-limiting illness, or if social/family circumstances or parental anxiety are significant, referral or close follow-up should be considered .
Children with fever without apparent source but with 'red' or 'amber' features should have investigations performed and be referred for specialist assessment .
Children with 'green' features and no amber or red features can be managed at home with safety netting advice, so referral is not routinely required unless clinical condition changes .

What criteria should I use to decide when to refer a patient with PUO to secondary care for further evaluation?